Hipot Test Theory
In order to understand what a Hot Hipot test is and how it is performed, it is first necessary to discuss the theory of the Hipot test itself. The Hipot test, sometimes called a Dielectric Withstand test, is used to verify the strength of the insulation between a product&#;s current-carrying components and its chassis or enclosure. This is done by applying a high voltage from the mains-input lines to the chassis of the product and measuring the resulting leakage current flowing through its insulation. The theory: if a voltage much higher than the product would normally see is applied across the insulation without a breakdown (which results in an excessive amount of leakage current flow), the product will be able to operate safely when run under nominal operating conditions.
The Hipot tester is used to indicate whether or not a dielectric breakdown of the insulation has occurred by monitoring the leakage current resulting from the applied test voltage. Even under normal operating conditions, some leakage current will be present in any device under test (DUT), but at minute and safe levels; however, when the insulation breaks down or is damaged an excessive amount flows to the chassis. This can present a substantial shock hazard to anyone that comes into contact with the product.
The Hipot test is so crucial because it is the best way to uncover workmanship and assembly defects in an electrical product that can lead to insulation breakdown. Mistakes during assembly or faulty/damaged components exist to an extent in any manufacturing environment, and the Hipot test can uncover units that are unfit and dangerous to sell. Some of the defects which could result in insulation breakdown include: pinched insulation, pinholes, and poorly crimped wiring. In order to detect for breakdown in electrical products, this test is usually performed during the manufacturing process on 100% of all manufactured units, as well as during routine repair and maintenance.
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A good rule of thumb for determining the test voltage during an AC Hipot test is to multiply the nominal input voltage (usually from a wall outlet given as an RMS voltage) by 2 and add volts.
AC Hipot test voltage = Nominal input voltage * 2 +
For a DC test use the following procedure to assure that the DC voltage is the same value as the peak of the AC waveform: multiply the calculated AC voltage by 1.414.
DC Hipot test voltage = AC Hipot test voltage * 1.414
By performing this operation, the DC voltage is applied at the same level as the peak of the AC voltage waveform.
The amount of time high voltage must be applied during testing is also specified in many safety agency standards. The most common test durations are 1 second for production tests and 1 minute for design tests. Further, agencies such as UL require that Hipot testers meet certain output voltage regulation specifications to ensure that the DUT is stressed at the correct voltage. Contact your local safety agency for more information about test duration and voltage requirements.
The Hipot test is set up by connecting the two output leads of the tester to the device under test. Follow the steps below to ensure that your tester is properly connected.
*By connecting the tester in this way, all of the internal current-carrying conductors are raised to the same potential with respect to the chassis. This connection scheme ensures that the high voltage waveform is applied directly across the insulation of the product.
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Hipot Test Shortcomings
The Hipot test has long been considered the most important electrical safety test; as such it is usually specified by safety agencies to be performed on all consumer and industrial products terminated in three- or two-pronged line cords. Historically this test has been effective on the gamut of electrical products due to a dependence on single-pole relays and mechanical switches. Yet products that operate off of a 220 volt input often incorporate double-pole relays that open both sides (line and neutral) of the input line. Further, with the dawn of the digital age we now find that many products incorporate electronic switches. Often these switches and relays cannot be closed manually without powering-up the product under test. In these cases a standard Hipot test becomes ineffective.
With both sides of the line open the Hipot tester cannot energize all the current-carrying conductors within the DUT and the test results become invalid. The only way to perform a valid Hipot test on products that contain these types of relays or electronic switches is to energize the product while the Hipot test is being performed. Yet in order to Hipot test a powered product, special steps must be taken since under normal conditions the line and neutral inputs of the DUT would be shorted together. This modified setup is commonly called a &#;Hot Hipot test.&#;
Hot Hipot Test Procedure
A Hot Hipot test is performed in the same fashion as a standard Hipot test. The primary difference is the addition of 1 piece of equipment, an isolation transformer. This transformer is used to isolate the input power to the DUT from earth ground. Without the use of this type of transformer, the chassis of the DUT, which is usually grounded, would be directly connected to the return of the Hipot tester (which is also referenced at or near ground potential). The return of the Hipot tester usually sees current in the milliamp range; however, without an isolation transformer the Hipot tester could be exposed to several amps of line current flowing back through its return. This could cause damage to the tester as well as create a possible shock or fire hazard during a Hot Hipot test.
The isolation transformer creates the necessary isolation between the input lines of the DUT and the Hipot tester. Of course, an AC test voltage is necessary for this test since DC waveforms don&#;t work with transformers. It is also important to verify that the isolation transformer is rated to handle the applied Hipot test voltage; this will prevent damage to the transformer.
Summary
With the advancement of the electronics industry Hot Hipot testing is becoming more and more common during routine production line testing. Products that were once operated solely through the use of mechanical relays and switches are now being controlled via electronic circuits that can only be energized while the product is running. Still other products that use 220 volt inputs contain relays that open both sides of the line, rendering a standard Hipot test ineffective. Whatever the reason, a working knowledge of the Hot Hipot test makes good sense of anyone working in the quality assurance or safety testing fields.
Although the Hot Hipot test has long been considered a mysterious and complex safety test, in actuality it isn&#;t much more difficult to perform than a standard Hipot test. With an understanding of the basic test procedure involved in performing a Hipot test and possession of the right equipment, a Hot Hipot test can be performed safely and efficiently. Paying attention to careful setup and implementation, a test operator, quality assurance supervisor, or engineer alike can feel comfortable performing a Hot Hipot test on a variety of products.
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